Multiaxis controlled motion knee brace with a four bar joint and method for producing same

ABSTRACT

A knee brace has a pair of femoral and tibial links and a four bar joint mechanism by which a lateral side one of the femoral links is pivotally connected to a lateral side one of the tibial links and a four bar joint mechanism by which a medial side one of the femoral links is pivotally connected to a medial side one of the tibial links. Each of the joint mechanisms comprises an inner, padded, pivot plate and an outer pivot plate, each of end of which is pivotally connected at a single point to a respective one of the femoral and tibial links. Furthermore, the pad on the inner pivot plate of the medial side joint mechanism carries a spherically cupped femoral condyle pad by which the brace, generally, and the joint mechanism, specifically, can be properly positioned relative to the knee of the wearer. The locations of the pivot points for the pivot plates on the links are set in accordance with parameters which are designed to produce a multiaxis motion of a reference point which will constrain the tibia to slide rearwardly relative to the femur in an initial range of flexion of the knee from a straight leg position and then to rotate relative thereto along an arcuate path. Additionally, the angle between imaginary lines through the pivots of each pivot plate is caused to be greater than a predetermined minimum value which will insure that the joint mechanism has sufficient strength not to break apart in use due to loads which are imposed horizontally across the knee joint.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to orthopedic devices for thestabilization and control of a human knee joint which has been injured.More particularly, the invention relates to a knee brace which willpermit the user a relatively high degree of freedom in the use of thebones while, at the same time, permitting control of the joint so as tooptimize healing and stability.

2. Description of Related Art

A knee brace of the initially mentioned type is disclosed in one of thepresent inventor's U.S. Pat. No. 4,890,607. In this patent, a multiaxiscontrolled motion knee orthosis utilizes a joint mechanism (which isimproved over one disclosed in U.S. Pat. No. 4,723,539 of one of thepresent inventors (Townsend)) having two camming slots and cam pinfollowers, wherein one camming slot is disposed in a transverse planeand serves to provide the anterior motion of an upper joint piece, whilethe second camming slot is disposed in a longitudinal orientation and toprovide a long arc segment for a unicentric phase of the jointarthrokinematics. During an initial range of motion, pivoting occursthrough a short arc segment about an upper cam pin within thelongitudinally extending arcuate slot. After the lower cam pin followerreaches the anterior end of the transverse slot, the lower cam pinfollower serves as an axis of rotation or pivot point for movement ofthe upper cam pin follower along the long arc segment of thelongitudinal slot.

Such an arrangement provides full control of the forceful action of thejoints throughout the entire range of motion while providing a joint ofhigh strength. Furthermore, in knee braces that are custom fit to aparticular user, the orthosis of this earlier patent has proved verysuccessful and has been free of significant problems. However, certainshortcomings have been encountered with regard to use of this dual pinand cam slot joint arrangement in less expensive knee braces intendedfor "off-the-shelf" use. In particular, a generic "off-the-shelf" unitmust be fitted by the doctor to a particular user and often involvesbending of the femoral and tibial links, such as by bending the femorallinks outwardly and the tibial links inwardly to fit someone with largerthan average thighs and smaller than average calves.

If such "doctoring" of an "off-the-shelf" unit results in theoverlapping surfaces of the joint not being exactly square, theserelatively large overlapping surfaces of the links are caused to bindagainst each other to an extent affecting the performance of the joint.More specifically, this binding produces excessive wearing of the joint,so that the critical tolerances necessary to proper control of the kneecan soon be lost. Furthermore, since, during each phase of movement, themotion of the joint is occurring about a single pivot, the full effectof the binding force is concentrated at the single pivot, and issufficiently great to cause the binding effect to be felt by the wearerto an undesirable degree.

At the other end of the spectrum, in top of the line knee braces,especially for use by professional athletes, the weight of the kneebrace is an important consideration, as is high resistance to wear. Oneway to reduce the weight of the joint, while increasing wear-resistancewould be to form the femoral and tibial links of a "space age"lightweight fiber and resin composite material. However, such materialsare expensive to machine and are very notch sensitive. Thus, a linkhaving slots as disclosed in the above-mentioned Townsend patents wouldbe cost-prohibitive to produce of a composite material, and would beprone to break apart due to the low notch strength of the compositematerial.

Thus, it would be desirable and advantageous to produce a joint whichwould have the benefits of the earlier Townsend designs without theirshortcomings. In particular, to have a knee brace with a joint thatcould be used in inexpensive "off-the-shelf" orthosis without resultingin the wearer feeling a significant binding effect or the jointexperiencing excessive wear due to binding. At the same time, such animproved knee brace should possess the capability to be manufactured oflightweight, wear-resistant composite materials.

Four bar linkages are also known for use in knee braces, even forproducing polycentric motion. For example, U.S. No. 3,901,223, disclosesa knee joint for orthopedic supports and splints using a four barlinkage in which a pair of different length, swinging links pivotallyinterconnect with bearing points on head portions formed on the ends offemoral (thigh) and tibial (shin) struts. These swinging links and theirrespective pivot points are designed so that, during flexion of thefemoral link relative to the tibial link from a fully extended positionof the joint, the longer, forward, link first pivots forwardly through agiven angle and then its motion reverses so that the forward link adoptsan identical position relative to the tibial link in the fully flexedattitude of the joint (approximately 134° degrees) as it held in thefully extended position. This movement is intended to simulate amovement of the knee in which the locus of the instantaneous centers ofrotation approximates a downward and forward curving path, beginningabout 3 inches up on the femur and ending at about the position of thefemoral epicondyles.

In U.S. Pat. No. 4,821,707 to Audette, a mechanical articulated jointfor a knee brace is shown which also uses a four bar type linkage in anattempt to produce a joint which will duplicate the complex motion ofthe knee; however, at best, the linkage as disclosed in this patent canonly do so in a most general way do to the approach taken therein.Furthermore, the design criteria outlined in this patent require thatthe shape of the condyle be known (which is difficult to do inpractice), require use of an arbitrarily set reference line segment andthe location of the point of tangency of this arbitrary line segment andthe condyle at three positions. As a result, an "off-the-shelf," genericknee brace is virtually impossible to produce in accordance with thispatent's teachings, and even achieving of a custom design brace thatforces the knee to follow a motion that correctly reproduces the propercomplex motion of a healthy knee is problematic.

Thus, there is still a need for controlled motion multiaxis joint for aknee brace which will meet the needs for both "off-the-shelf" and customtop-end knee braces, to an even greater extent than the cam and slotknee orthosis, mentioned above, being less prone to binding problems andbeing able to be made of composite fiber and resin materials; yet, atthe same time, still being able to constrain the tibia to sliderearwardly relative to the femur in an initial range of flexion of theknee from a straight leg position and then to rotate relative theretoalong an arcuate path.

SUMMARY OF THE INVENTION

In view of the foregoing it is a primary object of the present inventionto produce a knee brace having a multiaxis controlled motion knee jointwhich will sufficiently closely duplicate the type of complex slidingand rotating motion achieved, previously, only through cam pin and slottype joints in a four bar type joint which will be producible fromcomposite material when its lighter weight and improved wear resistanceoutweigh cost considerations and can be used in generic off-the-shelfknee braces without being subject to problems of unacceptably high wearor binding.

In keeping with the above object, it is a more specific object toprovide a four bar knee brace with a joint which will constrain thetibia to slide rearwardly relative to the femur in an initial range offlexion of the knee from a straight leg position and then to rotaterelative thereto along an arcuate path, yet will have sufficientstrength not to break apart in use.

It is yet another object of the present invention to provide a methodfor producing a knee brace fulfilling the preceding objects.

These and other objects and characteristics of the present invention areachieved in accordance with a preferred embodiment, wherein the kneebrace has a pair of femoral and tibial links and a four bar jointmechanism by which a lateral side one of the femoral links is pivotallyconnected to a lateral side one of the tibial links and a four bar jointmechanism by which a medial side one of the femoral links is pivotallyconnected to a medial side one of the tibial links. Each of the jointmechanisms comprises an inner, padded, pivot plate and an outer pivotplate, each end of which is pivotally connected at a single point to arespective one of the femoral and tibial links. Furthermore, the pad onthe inner pivot plate of the medial side joint mechanism carries aspherically cupped femoral condyle pad by which the brace, generally,and the joint mechanism, specifically, can be properly positionedrelative to the knee of the wearer.

The locations of the pivot points for the pivot plates on the links areset in accordance with parameters which are designed to produce amultiaxis motion of a reference point which will constrain the tibia toslide rearwardly relative to the femur in an initial range of flexion ofthe knee from a straight leg position and then to rotate relativethereto along an arcuate path. Additionally, the angle between imaginarylines through the pivots of each pivot plate is caused to be greaterthan a predetermined minimum value which will insure that the jointmechanism has sufficient strength not to break apart in use due to loadswhich are imposed horizontally across the knee joint.

These and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings which show, forpurposes of illustration only, a single preferred embodiment of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are schematic depictions for use in explaining designconsiderations in producing a joint mechanism in accordance with theinvention;

FIG. 5 is a flow diagram depicting an algorithm for use in arriving atthe locations for the pivot points of the joint mechanism of the presentinvention;

FIG. 6 is a vertical transverse cross section of a knee brace inaccordance with a preferred embodiment of the present invention;

FIGS. 7A through 16A are schematic depictions of the outside pivot plateconnection between the femoral and tibial links for illustrating theflexion movement thereof at various stages from a fully extended to afully flexed condition;

FIGS. 7B through 16B are schematic depictions of the inside pivot plateconnection between the femoral and tibial links illustrating the flexionmovement thereof at various stages from a fully extended to a fullyflexed condition;

FIG. 17 is a schematic view superposing the views of FIGS. 7A, 7Bthrough 16A, 16B, with the pivot plates removed for clarity, depictingthe motion of the femoral pivot points as the femoral link is swungthroughout the illustrated range of flexion motion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The design of a four bar joint mechanism requires a rational basis orcriteria for defining its design parameters since four pivot points areinvolved and their relative positions are infinitely variable, andwhenever these parameters are varied, unless at least three of themremain the same, a different mechanism results. Thus, before discussingthe specifics of a knee brace design utilizing a four bar jointmechanism, in accordance with the present invention, a rational designprocedure must be arrived at first.

In the case of the present invention, as a result of the experimentalanalyses which led to the cam mechanism of the above mentioned U.S. Pat.No. 4,890,607 (Townsend design), the which is hereby incorporated byreference, goal is to achieve a four bar joint mechanism which will,similarly, produce an anterior sliding movement of the femur relative tothe tibia followed by an essentially unicentric phase as the femur isflexed from its fully extended position. By an "essentially" unicentricphase it is meant that the motion produced by the four bar mechanismduring this phase is close enough to being a pivoting movement about astationary pivot point to be treated as such, bearing in mind thelimitations of a four bar mechanism to truly reproduce a single pivotmotion and the fact that a limited shifting of the pivot axis of thejoint mechanism (to the extent disclosed below) will not have an adverseimpact upon the wearer.

To arrive at a four bar joint mechanism which will produce thismovement, three positions of the mechanism must be arrived at. This isdone, in accordance with the invention, by specifying three positions ofthe femur relative to the tibia. The three positions are denoted as P₁,P₂ and P₃ in FIG. 1. These positions are defined by the translationalmotion of a point P (which is a reference point on the femur) and theangle of rotation of the femur from its initial position to the secondposition (α₂) and its angle of rotation from the second position to thethird position (α₃) As already mentioned, in accordance with theTownsend design, there are two phases of motion which constrain thedesign of the knee brace mechanism.

PHASE I

The first phase of motion is designed to move point P along a horizontalline from P₁ to P₂. The distance L₁ is a design parameter which can beadjusted to produce different design dimensions and provide a family ofbraces meeting different translational characteristics. This translationis produced as the femur rotates relative to the tibia through the angleα₂. This angle, also, is a design parameter. In the case of the Townsenddesign, L₁ is approximately 8-9 mm and the angle α₂ is the first 25° offlexion, and in accordance with this invention, these values are asindicated below.

PHASE II

The second phase of motion moves the reference point P from position P₂to position P₃. This motion is specified as another design parameter,the distance L₂ that is produced as the femur rotates through an angleα₃₋α₂.

Typical values for these design parameters which have been empiricallydetermined to be suitable are:

L₁ =7.5-10 mm

L₂ =0.0 (Points P₂ and P₃ are identical)

α₂ =25°-35°

α₃ =120°-135°

As the mechanism moves through its second phase of motion, the objectiveof the joint mechanism is to keep point P as close to P₂ as possible.Therefore, as the femur rotates through (α₃₋α2), the distance from thereference point P to P₂ is to be minimized in order to have an effectiveknee brace.

FORCE RESISTANCE

A second design requirement for the joint mechanism, in accordance withthe present invention, is that it be able to transmit a horizontal forceacross the knee joint for all angles of rotation of the femur, withoutdamage to itself, i.e., it will not break up. A typical loadingcondition is shown in FIG. 2.

In order to transmit the force F across the knee joint, the connectinglinks AD and BC must not be parallel. If they do become parallel, themechanism will be unable to resist a horizontal force. For practicalpurposes, the mechanism will become ineffective if the angle between thetwo connecting links (denoted as Φ in the FIG. 2) approaches someminimum value.

The value of the minimum angle Φ is a design parameter, and minimumvalues for Φ in the range of 24°-25° have been proved workable by thepresent inventors. Reasons for requiring a minimum value of Φ are thatthe mechanism, when built, will have some clearance in the joints; andthe materials in the mechanism will deflect under stress when the braceis loaded.

DESIGN EQUATIONS

The motion description for the knee brace mechanism defines a threeposition motion generation problem of the type described, generally, forfour bar linkages by Sander & Erdman, (ADVANCED MECHANISM DESIGN:Analysis and Synthesis, by George N. Sandor and Arthur G. Erdman,Prentice-Hall, Inc., Englewood Cliffs, New Jersey 07632, Volume 2,Chapter 2, Pages 92-97 and 122-125). The kinematic synthesis problem isdefined and the equations governing the design of 4-bar mechanisms areoutlined in this reference, which is hereby incorporated by reference tothe extent necessary to complete an understanding of this aspect of thepresent invention. FIG. 3 shows the general design problem (andcorresponds to FIG. 2.57 of Sander & Erdman). Point P₁, P₂ and P₃ arethe design locations noted earlier. The angles α₂ and α₃ are alsodefined earlier for the present knee brace design.

The design of a 4-bar mechanism is achieved by finding a set of vectorsW and Z which, when combined, form a mechanism as shown in FIG. 3. Theequations which control the design (Sandor and Erdman, Page 96) are:##EQU1## where

    δ.sub.2 =P.sub.2 -P.sub.1

    δ.sub.3 =P.sub.3 -P.sub.1

and β₂ and β₃ can be picked arbitrarily. Once two choices of β₂ and β₃are made, these equations define two sides of a four bar mechanism.

Instead of selecting the angles β₂ and β₃ arbitrarily, they aredetermined by selecting locations for the fixed pivot locations A and B.This procedure leads to the following equation for determining β₂ and asoutlined in Sandor and Erdman (page 122-124):

    D.sub.1 +D.sub.2 e.sup.iβ2 +D.sub.3 e.sup.iβ3 =0

where:

    D.sub.1 =R.sub.3 e.sup.iα2 -R.sub.2 e.sup.iα3

    D.sub.2 =R.sub.1 e.sup.iα3 -R.sup.3

    D.sub.3 =R.sub.2 -R.sub.1 e.sup.iα2

and for the side Z₂, W₂ :

    R.sub.1 =P.sub.1 -A

    R.sub.2 =P.sub.2 -A

    R.sub.3 =P.sub.3 -A

while for side Z₁, W₁ :

    R.sub.1 =P.sub.1 -B

    R.sub.2 =P.sub.2 -B

    R.sub.3 =P.sub.3 -B

D₁ is a known vector and, D₂ and D₃ are vectors of known magnitudes andunknown directions. The directions of the vectors which satisfy theequation lead to the values of β₂ and β₃. Knowing β₂ and β₃ allows thecalculation of W and Z as noted above.

The process just outlined will produce all possible mechanisms whichsatisfy any prescribed motion using the parameters P₁, L₁, L₂, α₂ andα₃. The free choices are the locations of the pivot points A and B. Todesign the joint mechanism for the knee brace in accordance with thepresent invention, the locations of A and B have been limited to alocating region that allows them to be positioned on the tibia andreference point P has been set at pivot C. This locating region isdefined by limits as shown in FIG. 4.

The limits are specified as "design parameters" in the flow diagram ofFIG. 5, which represents a method by which a suitable design definitioncan be arrived at, such as by using a computer. This locating region iscovered by a grid G and the point at each intersecting pair of gridlines is used as a possible selection of A and B. The spacing of thegrid lines in this region is a design parameter and has been set to 1-2mm by applicants (although not being shown as such in FIG. 4 forsimplicity). All combinations of points for the definition of A and Bare examined for possible mechanisms for the design of the knee brace.

The criteria used to select the best mechanism from all possiblesolutions are:

1. The maximum distance point P moves from design location P₂ as thefemur rotates from α₂ to α₃ is minimized.

2. The angle between line AD and BC must be greater than a specifiedminimum valve (typically Φ≧24.5°).

In addition, the distance between pivot A and pivot C must be greaterthan a specified minimum value (r_(min) =10 mm was used). This insuresthat there is sufficient material around these pivot locations to insurethat the brace is strong enough to resist the forces it must carry. Thisalso allows the mechanism to be constructed such that Links AD and BCare on opposite sides of the upper and lower struts (femoral and tibiallinks) of the brace.

With reference to the above criteria and FIG. 5, the manner in which thebest joint mechanisms can be selected, arrived at is clear. First, thedesign parameters are selected i.e., the grid is defined in terms ofx_(min), y_(min), x_(max), y_(max) (based upon the size of the tibia ofa leg the brace is to fit) and the grid interval (e.g., 1 mm). Then,sequentially each of the various possible pairs of grid intersectionpoints in the region are considered as locations for pivot points A andB. Then, using the above equations, corresponding values of β₂ and β₃and W₁, Z₁ and W₂, Z₂ are determined. Also, since it is desired to keeppoint P as close to P₂ as possible as the femur rotates through theangle between α₂ and α₃ during the second phase motion, in the nextstep, the maximum value ε of the difference between P and P₂ isdetermined, and for use in a subsequent step, the minimum value of Φ isdetermined. After determining these values, the value of ε just found issubtracted from the prior value of ε and if the result is negative, thisfirst test is failed and a new set of points is examined by returning tothe second step (for the first set of points this test will alwaysfail). If this test is passed by the latest value of ε being found to beless than the previous one, a second test is performed to determine ifthe joint would maintain an angle Φ above the prescribed minimum(24°-25°), and if not, again, a new set of points is examined byreturning to the second step. On the other hand, if the prescribedminimum angle is maintained, the process continues on to a third testwhich determines whether the distance between pivot A and pivot C isgreater than the specified minimum value (e.g., r_(min) =10 mm). If thislast test is passed, the design configuration defined by the current setof parameters is saved, and the process restarted at the second stepuntil the last of the possible locations for A and B have been tested,while if not, it is simply restarted.

Once all of the possible combinations of A and B have been exhausted,the last set of parameters saved represents the design configuration forproducing the best joint mechanism. As can be appreciated, thecriticality does not lie in the particular sequence of steps since, forexample, the order of the tests could be change without changing theresult. Likewise, while the Sandor & Erdman approach and equations havebeen utilized, the analysis and synthesis of the mechanism design can beperformed utilizing any other known approach and equations pertaining tofour bar linkages. What is of primary importance is the manner in whichsuch is applied to solution of the particular problems associated withthe present utilization of a four bar linkage to obtain a knee bracewhich will satisfactorily perform in accordance with the Townsend Designcriteria and will meet the requirements determined to be necessary bythe present applicants as set forth above.

KNEE BRACE

With the above in mind, a preferred embodiment of a knee brace meetingall of the objects and requirements of the present invention will now bedescribed with reference to FIGS. 6-17.

In FIG. 6, it can be seen that the knee brace 10 comprises a pair offemoral links 12a, 12b and a pair of tibial links 14a, 14b which are inthe form of a pair of upper struts and a pair of lower struts which canbe formed of aluminum, titanium, or fiber and resin composites. A cuffand/or one or more straps 16, of known design, are provided for holdingthe knee brace on the leg of a person requiring knee support (in thefigure, a brace for the right knee is shown and the left would bemirror-imaged relative thereto).

The lateral (outer) side femoral link, 12a is hinged to the lateral sidetibial link 14a via a four bar joint mechanism 18 and the medial (outer)side femoral link 14b is hinged to the medial side tibial link 12b via afour bar joint mechanism 20. The medial side joint mechanism 20 differsfrom the lateral side joint mechanism 18 only with respect to the padsprovided on their inner sides for protecting the wearer's leg. Thelateral side pad 22 is essentially flat on both sides, while the medialside pad 24 is larger and has a spherically-cupped shape on its sidethat faces the knee.

This spherically-cupped shape, while not essential, is advantageous inthat it allows a quick and easy positioning of the knee brace 10,especially the joint mechanism 18, 20 thereof, on the knee. Inparticular, by partially flexing the knee (for example, approximately 25to 35 degrees), the femoral condyle can be felt as a knob at the medialside of the knee and pad 24 can be placed on the femoral condyle as away of properly locating the joints mechanism so that they will becentered relative to the horizontal axis x_(c) passing through thefemoral condyle. In this way, by attaching the pads 22, 24 to supportplates 23, 25, respectively, which are swivelly connected to inner sidepivot plates 27 by, e.g., a rivet, they can remain essentiallystationary relative to the knee, as it is flexed and extended, therebyavoiding any discomfort to the wearer due to a rubbing of the jointmechanism against the side of the knee. However, it should beappreciated that it is not critical that an exact placement centered onaxis x_(c) be obtained.

Since the remaining details all apply equally to both of the jointmechanisms 18, 20, and to facilitate a side-by-side comparison of themovements at the inner and outer sides of the joint mechanisms 18, 20,they are shown as viewed from a common direction, i.e., as if the jointmechanism was transparent and equivalent to comparing the outer side ofthe lateral side joint mechanism 18 relative to the inner side of themedial side joint mechanism 20 and vice versa. Because of this approach,the "a" and "b" designations have been dropped from numerals 12 and 14in FIGS. 7-17.

Outer pivot plates 29 are each pivotally connected to a respectivefemoral link 12 and a respective tibial link 14, by a pivot pin that isformed, for example, by a rivet, so as to create first and second pivotpoints A, D. Each inner pivot plate 27 is similarly connected to createthird and fourth pivot points B, C.

While each of these pivot pins can be directly connected to a link 12,14, preferably, these connections are made with a plastic end cap 30that is fastened on the end of each link 12, 14 by fasteners 32, whichmay also be rivets. This form of attachment is advantageous in that itallows the joint mechanisms to be assembled and tested separately, andallows the assembled joint mechanisms to be inventoried and subsequentlyattached to any of variously shaped link struts, such as for legs havingsmall, medium or wide thighs and/or calfs. Furthermore, such a form ofattachment is especially advantageous when the link struts are formed offiber and resin composite materials since the cap 30 enclosing the endof a link 12, 14 formed of such a material will compensate for the lownotch strength of the composite material.

In the preferred and illustrated embodiment, the inner link plate 27generally resembles the shape of an ice cream cone and the outer linkplate 29 resembles a rectangle having rounded corners, one of which hasbeen removed. However, such shapes are not essential and can be variedso long as sufficient strength is retained and operation of the joint isnot otherwise adversely affected. Furthermore, with other pivot pointplacements, other shapes may be desirable or even necessary. As shouldbe apparent while called a four "bar" joint mechanism, such a mechanismdoes not literally require any of the links of which it is comprised tobe bar-shaped; by a four bar mechanism, it merely is meant that thejoint behaves in a manner which can be schematically represented by fourlinked bars.

Similarly, the configurations given to the end caps 30 have been chosen,on the one hand, to provide adequate material to securely hold the pivotpins and enable the intended movements of the joint to be obtained whilepreventing the end cap on femoral link 12 from contacting the end cap onthe tibial link 14, and on the other hand, to attach and coact with aset of extension stops R-0 through R-30 (FIGS. 7a, 7b to 12a, 12b) and aset of flexion stops F-60 through F-90 (FIGS. 13a, 13b to 15a, 15b), aswell as to inherently create a final flexion stop (FIGS. 16a, 16b).These stops serve to enable it to be possible to impose restrictions onthe permissible flexion and/or extension of the knee joint to insurethat a user cannot injure himself or herself by either extending orflexing beyond a desirable limit for that person due to injury ordeformity. However, the end caps 30 can be reconfigured to work withdifferently shaped motion stops and/or linkage modifications.

Turning, now, to the operation of this joint mechanism, point P of theabove calculations, as noted, has been chosen to correspond to pivotpoint C. Furthermore, while it would be desirable to locate pivot pointC, initially (i.e., in the 0° position of FIGS. 7a, 7b), on axis x_(c)of the femoral condyle, such is not essential. It has been found thatthe mechanism will work without problems with the location of pivot pinC situated up to 0.01 to 0.02 mm to the right and up to 3 mm to 5 mm upfrom axis x_(c) (as viewed in FIG. 7b), and thereby permittingadaptations to meet physical constraints of any particular knee brace.

The relative positions of the pivot links A-D and B-C, as the jointmechanism is flexed, can be seen in FIGS. 7-15 and show that the angle Φbetween them is always greater than 24°-25° throughout the full range offlexion. Furthermore, the initial location of point P, i.e., P_(o) isshown, in addition, in order to allow the movement of point P, withpivot point C, to be seen.

Furthermore, from the superposed view of FIG. 17, it can be seen thatpivot pin C executes an essentially linear movement corresponding to theanterior movement of the pivot cam pin within the linear cam slotproduced by the joint mechanism disclosed in the above-cited Townsenddesign. In actuality, analysis of the movement shows that a slightup-and-down movement occurs of less than 0.5 mm which, for purposes ofthe invention and actual usage, is insignificant, so that the motion canbe considered, nonetheless, an essentially linear movement. At the sametime, it can be seen that pivot pin D, initially remains essentially ina fixed location and then swings downwardly in an essentially unicentricarcuate motion that corresponds to that of the pivot cam which move inthe large arcuate slot in the joint mechanism of the Townsend design.Although, it should be recognized, as pointed out above, the motion isnot exactly unicentric; for example, despite the fact that pivot point Cis in the same location at the beginning and end of the phase 2 motion,it may shift up to about 2 mm, away and then back to this location, inthe interim. Of course, it should be recognized that the presentinvention is not limited to producing the exact same phases of movementas in the Townsend patent, as will be apparent from the permissiblerange of the values L₁, α₂ and α₃ set forth above. Furthermore, thejoint mechanism can be designed to produce more than two phases, orphase 2 could, itself, be subdivided into two or more separatesubphases.

Likewise, even though only a single embodiment has been shown anddescribed in accordance with the present invention, it is susceptible tonumerous changes and modifications as will be apparent to those skilledin the art. Therefore, the present invention is not limited to thedetails shown and described herein and instead, encompasses the fullscope of the appended claims.

We claim:
 1. In a knee brace for controlling movement of the femurrelative to the tibia during extension and flexion of a wearer's leg,having a pair of femoral links and a pair of tibial links, one of saidfemoral links being hinged to one of said tibial links by a jointmechanism at a medial side of the brace and the other of said femorallinks being hinged to the other of said tibial links by a jointmechanism at a lateral side of the brace; wherein each said jointmechanism is a four bar linkage having a first pivot plate pivotallyconnected at a first pivot point to a respective one of said tibiallinks and at a second pivot point to a respective one of said femorallinks, and a second pivot point, pivotally connected at a third pivotpoint to a respective one of said tibial links and at a fourth pivotpoint to a respective one of said femoral links, and wherein therelative positions of said pivot points are adapted to produce a motionof a reference point on the femoral condyle, which is an essentiallylinear, horizontal translation, with reference to an orientation of thebrace in which the tibial and femoral links are vertically oriented in astraight leg position, that results in the tibia being constrained toslide posteriorly relative to the femour in a first phase of flexionfrom said straight leg position, and which is an essentially unicentricarcuate movement about said reference point in a second phase offlexion; and wherein an angle of intersection defined between a portionof an imaginary line drawn through said first and second pivot pointsand a portion of an imaginary line drawn through said third and fourthpivot points, said portion extending from said said first and thirdpivot points to a point of intersection of said imaginary lines is atleast 24° throughout the full range of flexion from said straight legposition to a fully flexed position.
 2. Knee brace according to claim 1,wherein the horizontal translation of said reference position duringsaid first phase is 7.5 mm to 10 mm, and said reference point remainswithin 2 mm of its position at the end of said first phase during saidsecond phase.
 3. Knee brace according to claim 2, wherein said firstphase extends from 0° of flexion to 25° to 35+ of flexion, and saidsecond phase extends from the end of the first phase to 120° to 135°degrees of flexion.
 4. Knee brace according to claim 1, wherein saidfirst phase extends from 0° of flexion to 25° to 35° of flexion, andsaid second phase extends from the end of the first phase to 120° to135° degrees of flexion.
 5. Knee brace according to claim 4, wherein,one of the pivot plates of each joint mechanism is located on an innerside of the femoral and tibial links, and the other of said pivot platesis an outer side of the femoral and tibial links.
 6. Knee braceaccording to claim 1, wherein a cushion is swivelly mounted to each ofthe inner side pivot plates, at the medial side of the knee brace, thecushion is spherically cupped for fitting on the femoral condyle of thewearer.
 7. Knee brace according to claim 6, wherein a cushion isswivelly mounted to each of the inner side pivot plates, at the medialside of the knee brace, the cushion is spherically cupped for fitting onthe femoral condyle of the wearer.
 8. Knee brace according to claim 1,wherein said reference position is on the femoral condyle in proximityto one of said second and fourth pivot points, and wherein the distancefrom said one of the second and fourth pivot points to the respectiveone of the first and third pivot points of the same pivot plate is atleast 10 mm.
 9. Knee brace according to claim 1, wherein each of saidfemoral and tibial links comprises strut and an end cap secured over anend of the strut; and wherein said pivot points are located on said endcaps.
 10. Knee brace according to claim 9, wherein the end caps areformed of plastic and said struts are formed of a fiber and resincomposite material.
 11. In a knee brace for controlling movement of thefemur relative to the tibia during extension and flexion of a wearer' sleg, having a pair of femoral links and a pair of tibial links, one ofsaid femoral links being hinged to one of said tibial links by a jointmechanism at a medial side of the brace and the other of said femorallinks being hinged to the other of said tibial links by a jointmechanism at a lateral side of the brace; wherein each of said femoraland tibial links comprises a strut and an end cap secured over an end ofthe strut, the end caps being formed of plastic and said struts beingformed of a fiber and resin composite material; wherein each said jointmechanism comprises pivot means, interconnecting a portion of the endcap of the tibial link which extends axially beyond an end of the strutover which it is secured and a portion of the end cap of the femorallink which extends axially beyond an end of the strut over which it issecured, for producing a motion of a reference point on the femur whichis an essentially linear, horizontal translation that results in thetibia being constrained to slide posteriorly relative to the femur in afirst phase of flexion from a straight leg position, and which producesan essentially unicentric arcuate movement about said reference point ina second phase of flexion.
 12. Knee brace according to claim 11, whereinthe horizontal translation of said reference position during said firstphase is 7.5 mm to 10 mm, and said reference point remains within 2 mmof its position at the end of said first phase during said second phase.13. Knee brace according to claim 12, wherein said first phase extendsfrom 0° of flexion to 25° to 35° of flexion, and said second phaseextends from the end of the first phase to 120° to 135° degrees offlexion.
 14. Knee brace according to claim 11, wherein said first phaseextends from 0° of flexion to 25° to 35+ of flexion, and said secondphase extends from the end of the first phase to 120° to 135° degrees offlexion.
 15. Knee brace according to claim 11, wherein each said pivotmeans comprises a four bar linkage having a first pivot plate pivotallyconnected at a first pivot point to a respective one of said tibial linkend caps and at a second pivot point to a respective one of said femorallink end caps, and a second pivot plate, pivotally connected at a thirdpivot point to a respective one of said tibial link end caps and at afourth pivot point to a respective one of said femoral link end caps;and wherein an angle of intersection between an imaginary line drawnthrough said first and second pivot points and an imaginary line drawnthrough said third and fourth pivot points is at least 24° throughoutthe full range of flexion from said straight leg position to a fullyflexed position.